Recently, seedlings of vegetables have been cultivated by producers as a specialized profession separate from usual farmers. As a result, so-called "plugged seedlings" are supplied by the cultivators of seedlings to farmers who work exclusively to plant the supplied plugged seedlings in fields. Such a division of labor has been widely spread. Moreover, farmers in some areas often carry out mechanical planting by setting machines in order to save labour in planting plugged seedlings.
A semi-automatic one has been well known as such a setting machine. This setting machine includes a multiple ridge transplantation trolley dragged by a tractor. On the trolley there are provided a support base for seedling culture trays for supplying seedlings, sets of rotary hoppers operated synchronously with rotation of wheels of the trolley, and transplantation opening members connected with the rotary hoppers. The transplantation opening members periodically penetrate into the ground when the trolley runs. In using the setting machine constructed as above, workers who supply plugged seedlings sit on seats corresponding to respective transplantation ridges in a field, while seedling culture trays are manually supplied in front of these workers by other workers for supplying the trays. Thereafter, plugged seedlings are manually taken out of the trays upwardly and supplied into the hoppers by the first-mentioned workers. The plugged seedlings are forced to penetrate into the ground to be planted by operating the transplantation opening members synchronously with rotation of the wheels of the trolley. In this case, however, there has arisen the following problem. That is, the planting operation is not performed completely automatically, and requires manual operations co-operating with each other which are to be carried out by a tractor operator, one or two workers for supplying the seedling culture trays, and six workers for supplying the plugged seedlings, hence eight or nine workers as a whole. Therefore, it becomes difficult to accomplish improvement in operation efficiency and saving of personal expenses. A highly automatized setting machine has thus long been requested in this industrial field.
In order to fulfill such requirement, there has been proposed a setting machine which is to apply a rice transplanter mechanism. This setting machine includes a support base for supporting seedling culture trays, and a mechanism for pulling seedlings out of the trays on the support base by transplantation pawls to transplant them into the field. This system is realized by the use of seedling culture trays made of paper materials, which are composed of pieces of cardboard arranged in lattice to form a plurality of cells. In this case, after the seedling in each cell has been pulled and separated from the tray by transplantation pawls together with cardboard pieces defining the cell, the seedling is inserted into the ground to be transplanted. However, as the prior art setting machine uses the trays made of paper material, adjacent seedlings often grow whose roots entangle with each other across walls of the cells. As a result, when the seedlings are separated from the paper tray pieces by the transplantation pawls, roots of the seedlings are often torn off or damaged, permitting viruses to invade the roots. Moreover, there is a risk that growth of the roots of the seedlings is detrimentally affected by the cardboard pieces set in the ground together with the seedlings.
Seedling culture trays made of synthetic resins have been known, including independent cells to prevent roots of adjacent seedlings from entangling with each other. However, such seedling culture trays have been used only in completely manual operation or semiautomatic operation. In more detail, different from the paper trays, the cells of the trays made of synthetic resin could not be separated from the trays by pulling them by means of the transplantation pawls. Therefore, it has been considered that completely mechanical transplanting operation with synthetic resin trays is practically impossible or very difficult.